1. Field of the Invention
This invention relates to a line-of-sight (LOS) processor architecture for a receiver for use in semi-active or active pulsed electromagnetic (EM) targeting systems, and more particularly to an all-digital architecture that addresses the size, weight and power constraints of typical applications and provides a platform for enhanced techniques for sensitive pulse detection over a wide field-of-view, adaptive pulse detection, LOS processing and counter measures.
2. Description of the Related Art
Certain aircraft, missiles and smart projectiles include a receiver that allows the platform to receive and process EM pulses, typically in the near IR, transmitted by a known source and returned off of a target. The receiver may be fixed to look along the line-of-sight (LOS) of the platform or gimbaled to look along a receiver LOS relative to the platform LOS. If the source is remotely located the system is referred to as ‘semi-active’ whereas if the source is co-located on the platform with the receiver the system is referred to as ‘active’. The receiver may perform many different functions including acquisition and tracking, identification, active countermeasures and aimpoint selection. A core capability required to support each of these functions is to reliably and accurately detect the presence of EM pulses returned from a real target from amidst natural clutter and active jamming and to measure the line-of-sight (LOS) from the platform to the target. Errors in detection or LOS processing can cause mission failure.
As shown in FIG. 1, a typical receiver 10 includes a quad detector 12 that detects incident radiation (e.g. transmitted pulses reflected off a target, pulses transmitted to actively jam the receiver and background noise) and generates analog signals 14, a bank of pre-amps 15 that amplify the analog signals 14 and a LOS processor 16 that detects “pulses”, calculates a LOS to a target and performs additional processing to generate a target report 18 that is passed on to a command and guidance processor 20 to guide the platform to the target.
LOS processor 16 is based on a mixed analog/digital architecture and functionality. The processor can be roughly divided into three functional areas: pulse detection 22, LOS calculation 24 and DSP post-processing on the pulses and LOS 26. Pulse detection 22 is a standard analog operation in which an analog sum channel is thresholded to satisfy a Constant False Alarm Rate (CFAR) criteria evaluated over a fixed period (typically multiple EM pulse repetition intervals for the known source) to identify candidate pulses. Parameters such as rise/fall times and pulse width are extracted from each candidate pulse and evaluated to independently determine whether the candidate pulse is a real target pulse. Upon validation of a target pulse, a sample and hold circuit samples the analog signals once near the peak and analog circuits combine the peak values to provide the LOS calculations 24. Ideally, the analog signals are sampled at the peak but there is some degree of alignment error between peak detection and the delayed analog signals. The DSP post-processing 26 performs time-correlation on the validated pulses to determine if they occur at the expected repetition rate of the transmitted EM pulses and to determine if the LOS calculations are acquiring and tracking a target.